This invention relates to protection circuits in modulated focus voltage amplifiers for television apparatus.
The scanning distance of cathode ray tubes from the center of deflection to the scanned raster is not uniform. The scanning distance varies markedly as the beam is deflected horizontally and vertically from the central portion of the screen, and consequently, defocusing is most severe at the corners of the raster. The adoption of picture tubes of increased size and wider deflection angles in recent years has emphasized the defocusing effect at the corners of the scanning pattern.
Efforts have been made to control focus in electrostatically focused tubes by changing the magnitude of the focus potential with displacement of the beam from the center of the raster, that is, by dynamic focusing. A unidirectional potential to the focus electrode of the electrostatically focused cathode ray tube establishes the proper focus condition at the center of the raster and the concurrent application of a suitable varying potential maintains that condition throughout the scanning raster.
In a dynamic focus circuit for a CRT system having electrostatic focus, it can be necessary to apply to the focus electrode about 800 volts p--p parabolic correction at a horizontal rate and about 200 volts p--p parabolic correction at a vertical rate. The horizontal rate correction is typically developed across the secondary of a step up transformer terminated in a capacitor. The vertical rate correction is added in series with the horizontal correction, typically by means of an amplifier that operates, for example, from a +250 volt power supply. In order to achieve a symmetrical response with minimum bias power, a class A B amplifier can be used that employs a low impedance pull-up to charge the load capacitor. This type of push pull amplifier is subject to catastrophic failure if both devices turn on at the same time, such that a direct short occurs between the +250 volt supply and ground.
The problem can be addressed by providing an R-C decoupling stage between the amplifier and the +250 volt source, such that there is minimum DC voltage drop during normal operation and a large protecting drop during a failure. The short term transients are bypassed via the filter capacitor. The capacitor needed for such a decoupling stage is large, for example 0.33 .mu.f rated at 300 volts, and expensive. The series resistor of the R-C filter stage also causes a problem. The series resistor introduces a voltage loss in normal operation on the order of 20 volts, that limits the swing of the amplifier output.